1. Field of the Invention
The present invention relates to a charging control method and apparatus for a power generation system and, more particularly, to a charging control method and apparatus for a power generation system used together with a DC power source having an unstable output and an accumulation cell for stabilizing the unstable DC power source.
2. Related Background Art
FIG. 9 shows the circuit arrangement of a charging control apparatus for a power generation system. Referring to FIG. 9, the apparatus includes a solar cell 41 as an unstable power source, a reverse current prevention diode 42, a control unit 43, an accumulation cell 44, a load connection switch 45 (or element), a load 46, an overcharging prevention short-circuit switch (or element) 47, which is opened/closed by the control unit 43, and a detection unit 48 for detecting an output voltage from the accumulation cell 44. A short-circuit control signal 49 is input from the control unit 43. A detection signal based on the terminal voltage of the accumulation cell 44 which is detected by the detection unit 48 is input to the control unit 43.
FIGS. 10A, 10B, and 10C are timing charts showing the operation of the apparatus shown in FIG. 9. FIG. 10A shows an output current from the solar cell; FIG. 10B, the terminal voltage of the accumulation cell; and FIG. 10C, the connected state of the load. In these charts, a voltage e is a predetermined voltage at which a charging operation is inhibited, and a voltage f is a predetermined voltage at which a charging operation is permitted. In the timing charts, time t.sub.41 is the time at which the terminal voltage reaches the predetermined voltage e, i.e., the time at which inhibition of charging is started; time t.sub.42, the time at which load connection is started; t.sub.43, the time at which the terminal voltage reaches the predetermined voltage f, i.e., the time at which inhibition of charging is canceled; and time t.sub.44, the time at which load connection is canceled.
Referring to FIG. 9, the solar cell 41 receives sunlight and generates an electromotive force to output a current. The current output from the solar cell 41 is stored in the accumulation cell 44 via the reverse current prevention diode 42. It is assumed in this case that the overcharging prevention short-circuit switch (or element) 47 is closed by the short-circuit control signal 49 from the control unit 43. Assume that this state corresponds to the state at recording start time t.sub.40 in FIGS. 10A to 10C. In this case, the accumulation cell 44 is in a charging state, and the terminal voltage of the accumulation cell 44 rises with the lapse of time. This state corresponds to a curve b.sub.41 in FIG. 10B.
This voltage is detected by the detection unit 48, and the resultant detection signal is loaded into the control unit 43. At time t.sub.41 at which a voltage b of the accumulation cell 44 rises, owing to a charging operation, to the predetermined voltage e at which the charging operation must be terminated, a short-circuit control signal 49 is output from the control unit 43 to close the short-circuit switch (or element) 47 for preventing an overcharging operation. When the short-circuit switch (or element) 47 is closed, the output of the solar cell 41 is grounded, and the output current from the solar cell 41 is discharged to the ground. As a result, charging of the accumulation cell 44 is stopped.
A curve b.sub.42 in FIG. 10B represents a voltage drop due to self-discharging; and a curve b.sub.43, a voltage drop due to discharging to the load 46. In this background art, the condition for permitting a charging operation again is that the terminal voltage of the accumulation cell 44 drops to the predetermined voltage f. This predetermined voltage f is set to be a value lower than the above-mentioned predetermined voltage e corresponding to the characteristics of the accumulation cell 44 used or the like. In addition, the predetermined value f is selectively set in consideration of prevention of oscillation of a control system circuit for preventing an overcharging operation. Therefore, when the terminal voltage of the accumulation cell 44, whose charging operation is inhibited, is set between the predetermined voltage e and the predetermined voltage f, even if the load 46 is connected, all the output currents from the solar cell 41 are discharged to the ground via the overcharging prevention short-circuit switch (or element) 47. The above background art has the following problems.
(1) When a charging operation is inhibited, even if extra currents are generated by the solar cell 41, all such extra currents are discharged to the ground, and the power generated by the solar cell 41 is not effectively used. PA1 (2) When charging of the accumulation cell 44 is inhibited, and the load 46 is connected, since only power from the accumulation cell 44 is supplied to the load, an unnecessary discharging operation is performed. This unnecessary discharging operation reduces the warranted non-charging period in number of days of the system, thus shortening the service life of the accumulation cell 44.